Narrative:

We crossed over ZZZ at 5000 ft; with all engine indications normal; but at about 5 mi east of the airport; the engine made a loud pop; and began running rough and vibrating. I checked and enriched the mixture; checked fuel status and throttled back the power to lessen the vibrations. Having passed over ZZZ; I knew that it was the closest airport from my position. I declared an emergency with ZZZ approach. ZZZ approach gave me a turn and vectors to the airport for landing. ATC asked what the emergency was; souls on board and fuel status. I reported the rough running engine; and that there were 2 souls and 2 hours of fuel remaining; with fuel gauges indicating about a 1/2 tank for both left and right side tanks. I had plenty of altitude for the glide back to ZZZ and kept the engine running at minimal RPM to minimize the vibration; yet provide power if needed. Approach cleared me for landing; and then passed me off to the tower controller. The tower controller again cleared me to land and notified me that the 'emergency equipment was rolling' (as a former firefighter; I appreciated knowing that equipment would be standing by). As I turned for final approach; I added flaps to help increase the descent rate; but with surface winds as strong as they were; I had to raise flaps and add a touch of power to maintain the GS to the runway. The plane touched down on the displaced threshold portion of the runway; and just short of the main runway threshold; which was quite suitable for a light aircraft. As I rolled to a stop; the engine; which was idling at minimal RPM; stopped. The tower controller asked me to turn off at the next taxiway; and taxi to the GA ramp; but I was unable to coast clear of the runway. I made an attempt to restart the engine; but since the engine seemed to be straining to turn over; I notified the tower controller that I would need a tow to the ramp. After we towed the plane back to the FBO; we noted that oil was dripping from the engine cowling onto the nosewheel. It was not a steady flow; but a slow drip; just covering the nosewheel fairing; but not enough to make a puddle on the ramp. After parking and securing the plane; I pulled off the cowling; and traced the source of the oil back to the air-intake box under the carburetor. The oil was dripping from a small hole in the bottom of the 'box;' making me believe that the source of the oil was from a cylinder with an 'open' or otherwise stuck intake valve. I also checked the fuel in the tanks; by dipping each side; with 12 (of 19) gals in the right tank and 11 (of 19) gals in the left tank; which indicated to me that fuel quantity was not an issue. I arranged to meet with a mechanic from the FBO the next day. We found that the rear right cylinder (#3 on the lycoming 320-E2D engine) was cold. Pulling the bottom spark plug; it was oiled fouled. A thumb compression test showed that there was no compression for this cylinder. Pulling the top spark plug revealed that the top plug was damaged; and that there was a partial ring of metal in the cylinder; just beyond the spark plug barrel.

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Original NASA ASRS Text

Title: CESSNA C172 PLT HAS A ROUGH RUNNING ENG; DECLARES AN EMER AND DIVERTS FOR LNDG.

Narrative: WE CROSSED OVER ZZZ AT 5000 FT; WITH ALL ENG INDICATIONS NORMAL; BUT AT ABOUT 5 MI E OF THE ARPT; THE ENG MADE A LOUD POP; AND BEGAN RUNNING ROUGH AND VIBRATING. I CHKED AND ENRICHED THE MIXTURE; CHKED FUEL STATUS AND THROTTLED BACK THE PWR TO LESSEN THE VIBRATIONS. HAVING PASSED OVER ZZZ; I KNEW THAT IT WAS THE CLOSEST ARPT FROM MY POS. I DECLARED AN EMER WITH ZZZ APCH. ZZZ APCH GAVE ME A TURN AND VECTORS TO THE ARPT FOR LNDG. ATC ASKED WHAT THE EMER WAS; SOULS ON BOARD AND FUEL STATUS. I RPTED THE ROUGH RUNNING ENG; AND THAT THERE WERE 2 SOULS AND 2 HRS OF FUEL REMAINING; WITH FUEL GAUGES INDICATING ABOUT A 1/2 TANK FOR BOTH L AND R SIDE TANKS. I HAD PLENTY OF ALT FOR THE GLIDE BACK TO ZZZ AND KEPT THE ENG RUNNING AT MINIMAL RPM TO MINIMIZE THE VIBRATION; YET PROVIDE PWR IF NEEDED. APCH CLRED ME FOR LNDG; AND THEN PASSED ME OFF TO THE TWR CTLR. THE TWR CTLR AGAIN CLRED ME TO LAND AND NOTIFIED ME THAT THE 'EMER EQUIP WAS ROLLING' (AS A FORMER FIREFIGHTER; I APPRECIATED KNOWING THAT EQUIP WOULD BE STANDING BY). AS I TURNED FOR FINAL APCH; I ADDED FLAPS TO HELP INCREASE THE DSCNT RATE; BUT WITH SURFACE WINDS AS STRONG AS THEY WERE; I HAD TO RAISE FLAPS AND ADD A TOUCH OF PWR TO MAINTAIN THE GS TO THE RWY. THE PLANE TOUCHED DOWN ON THE DISPLACED THRESHOLD PORTION OF THE RWY; AND JUST SHORT OF THE MAIN RWY THRESHOLD; WHICH WAS QUITE SUITABLE FOR A LIGHT ACFT. AS I ROLLED TO A STOP; THE ENG; WHICH WAS IDLING AT MINIMAL RPM; STOPPED. THE TWR CTLR ASKED ME TO TURN OFF AT THE NEXT TXWY; AND TAXI TO THE GA RAMP; BUT I WAS UNABLE TO COAST CLR OF THE RWY. I MADE AN ATTEMPT TO RESTART THE ENG; BUT SINCE THE ENG SEEMED TO BE STRAINING TO TURN OVER; I NOTIFIED THE TWR CTLR THAT I WOULD NEED A TOW TO THE RAMP. AFTER WE TOWED THE PLANE BACK TO THE FBO; WE NOTED THAT OIL WAS DRIPPING FROM THE ENG COWLING ONTO THE NOSEWHEEL. IT WAS NOT A STEADY FLOW; BUT A SLOW DRIP; JUST COVERING THE NOSEWHEEL FAIRING; BUT NOT ENOUGH TO MAKE A PUDDLE ON THE RAMP. AFTER PARKING AND SECURING THE PLANE; I PULLED OFF THE COWLING; AND TRACED THE SOURCE OF THE OIL BACK TO THE AIR-INTAKE BOX UNDER THE CARB. THE OIL WAS DRIPPING FROM A SMALL HOLE IN THE BOTTOM OF THE 'BOX;' MAKING ME BELIEVE THAT THE SOURCE OF THE OIL WAS FROM A CYLINDER WITH AN 'OPEN' OR OTHERWISE STUCK INTAKE VALVE. I ALSO CHKED THE FUEL IN THE TANKS; BY DIPPING EACH SIDE; WITH 12 (OF 19) GALS IN THE R TANK AND 11 (OF 19) GALS IN THE L TANK; WHICH INDICATED TO ME THAT FUEL QUANTITY WAS NOT AN ISSUE. I ARRANGED TO MEET WITH A MECH FROM THE FBO THE NEXT DAY. WE FOUND THAT THE REAR R CYLINDER (#3 ON THE LYCOMING 320-E2D ENG) WAS COLD. PULLING THE BOTTOM SPARK PLUG; IT WAS OILED FOULED. A THUMB COMPRESSION TEST SHOWED THAT THERE WAS NO COMPRESSION FOR THIS CYLINDER. PULLING THE TOP SPARK PLUG REVEALED THAT THE TOP PLUG WAS DAMAGED; AND THAT THERE WAS A PARTIAL RING OF METAL IN THE CYLINDER; JUST BEYOND THE SPARK PLUG BARREL.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.